1. Field of the Invention
The present invention relates to techniques for manufacturing stranded wires and cable conductors using oxide superconductor, and more particularly, to techniques for providing an electrically insulating material suitable for the step of sintering oxide superconductor, and providing a stranded wire and a cable conductor which have high critical current, using such an electrically insulating material.
2. Description of the Background Art
Since the oxide superconducting wire exhibits superconductivity at liquid nitrogen temperature, its application to superconducting cables and the like has been expected and its development has been ongoing. Since a wire having a bismuth-based 2223-phase oxide superconductor sheathed with silver in particular can be readily formed long, and a relatively high critical current density (Jc) results, the research and development is proceeding.
When an oxide superconducting wire is directed to alternating current (ac) application, a problem of ac losses is encountered. Particularly when such a wire is applied to a power cable, a loss by a self-magnetic field is problematic. It is known that the loss by self-magnetic field is effectively reduced by employing the structure in which wires that form the power cable are translocated. Translocation of the wires may be achieved by twining the wires. In the process of twining the wires, however, great bending strain is loaded upon the wires, and therefore it has been quite difficult to twine the wires having relatively fragile oxide superconductor, without reducing the superconducting property. In order to obtain significant effects by twining the wires, it is also important to provide the wires with electrical insulation. In a stranded wire using a metal-based superconductor, for example, the strands are coated with enamel. In the field of Nb.sub.3 Sn application, glass is used as an insulating material. When a Nb.sub.3 Sn superconducting coil is manufactured by the wind-and-react process, for example, wires coated with glass are wound into a coil and heat-treated.
The cable conductor in which tape-shaped silver-sheathed bismuth-based superconducting wires are spirally wound on a former in layers is generally known. Such a structure, however, can easily generate current drift.